The present invention relates to level sensors of the type used in the process control industry. More specifically, the present invention relates to capacitive level sensors which provide an output to, and receive power from, same two wire process control loop (can be relay output as well).
Process control loops are frequently used in industrial processes to control the process. One type of sensor that can be coupled to such a loop is a level sensor, and specifically a capacitive based liquid level sensor. Such sensors detect a wet or a dry condition by measuring the capacitance between a test probe (or sensor rod) and electrical ground. When the test probe is covered with liquid, an RF signal is more easily conducted to electrical ground. This is detected and an output is provided indicating a wet condition.
However, over the life of the sensor, it is known that coatings, and in particular conductive coatings, can build up on the sensing rod and lead to inaccurate readings. One known solution is to provide a shield which partially surrounds the sensing rod and is driven to the same electrical potential as the sensing rod. This creates an electrically neutral zone between the shield and the sensing rod to thereby reduce the effect of a conductive coating on the sensor. Various capacitive liquid level sensors are shown in Geiger, U.S. Pat. No. 4,347,741 issued Sep. 7, 1982, Kuhlman, U.S. Pat. No. 4,515,015 issued May 7, 1985 and Marsh et al. U.S. Pat. No. 5,048,335 issued Sep. 17, 1991.